The present invention relates, in general, to a high-reflection silver mirror, and specifically, to an environmentally stable silver mirror having very high reflection values over a wide spectral range.
Silver is a material with excellent optical properties. It is used inter alia in the production of highly reflecting mirrors and, in particular, for the production of elements for regulating solar energy transmission, such as architectural glass coatings, heat protection layers, automobile windows and the like. The use of silver as an optical thin-film material is extensively described for example in the publication "Thin-Film Optical Filters", H. A. Macleod, Adam Hilger Ltd., Second Edition. Unfortunately, silver has poor environmental compatibility, since it is, on the one hand, relatively soft and consequently can readily be mechanically damaged and, on the other hand, an impairment of optical properties occurs due to corrosion if the silver mirror is exposed, without protection against the environment or specific media. Silver sulfides, for example, are formed.
For this reason silver layers are frequently packed in layer systems wherein the material selected for the remaining layers is determined, on the one hand, by the desired optical properties such as spectral properties, as well as, on the other hand, by the necessity of increasing the resistance of the silver layer relative to environmental influences. A consideration of the nucleation properties of silver on these materials is also of significance, however.
Oxides, zinc sulfide, nitrides or metals are frequently used in order to pack silver. In particular, oxides are used due to their optical properties and their resistance and also because of their hardness. Applying the oxide layer, however, can already cause a degradation of the silver which is the reason why, in prior art, measures have been suggested in order to avoid this problem.
For example, DE-OS-33 07 661 suggests first covering the silver layer with a further metal layer comprises aluminum, titanium, tantalum, chromium, manganese or zirconium, onto which further metal layers, and lastly an oxide layer, are disposed, comprising indium oxide, tin oxide or a mixed oxide thereof. DE-OS-35 43 178 suggests a multilayer covering wherein the silver layer, in turn, is covered by a further metal layer comprising tantalum, tungsten, nickel or iron, which further metal layer, in turn, is covered by an oxide layer, wherein SnO, SiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5 or ZrO.sub.2 are suggested as the Oxide layers.
In analogous manner, U.S. Pat. No. 3 682 528 suggests covering the silver layer with a thin nickel layer, should further layers be applied.
According to another variant, DE 30 27 256 suggests applying at least one hypostoichiometric oxide layer on the silver, comprising, for example, titanium oxide or titanium nitride or a mixture thereof.
Lastly, DE-A-33 29 504 suggests covering the silver layer with a dielectric layer wherein the material composition in the region of the transition areas, changes gradually and continuously. Titanium oxide is mentioned, for example, as such a dielectric layer.